This is one of the oldest organ stops, first appearing in the late 1400's.
It has varied in construction and timbre over the centuries.
While it takes its name from the instrument of the same name,
a capped reed with a curved body and a muffled, buzzing tone,
the organ stop in its most familiar
form has a tone resembling that of the Clarinet,
though that tone did not develop, according to Williams, until the early 1800's.

In the classical French organ (1650-1790) it was the reed
on the Positif: while the Positif might contain other reeds, and
other divisions might contain Cromornes, no French classic
organ of any size lacked one in its Positif. Here it was used as
both a solo and a combinational stop; it was often specified in the
titles of compositions, and it dominated the Grand Jeu of the Positif.
It was usually made with narrow cylindrical half-length resonators.
According to Dom Bedos, the Cromorne was seldom used in the Grand Orgue,
but usually appeared in the Positif, but usually only in small instruments
which did not have enough space for a Trompette, and rarely
in large instruments. This contradicts other evidence, which shows
that the Cromorne was an essential tonal ingredient of the
classical French organ.

Wedgwood reports that the name Krummhorn was used for Clarinet
stops which extended below tenor C, at a time when the Clarinet stopped
at tenor C.
Locher claims that it has a soft horn tone, and Audsley quotes Seidel
as saying:

Properly ‘Cormorne’, from cor ‘horn’ and morne ‘mournful,
still, soft’, signifying a soft, quiet Horn, is a lingual stop of a
delicate intonation, of 8 ft. or 4 ft. pitch, of tin or pipe metal,
open or shaded, and sometimes formed of small-scaled cylindrical pipes.

Clearly, neither Locher nor Seidel were describing the French classical stop.
Bonavia-Hunt describes it as a loud, broad-toned Clarinet.
The name Cremona is a corruption, and has other meanings.

Hopkins & Rimbault give a good example of how much knowledge regarding the Baroque
had been lost by the Victorian period. That source claims:
“The name Krumm-horn ... signifies a Cornet or small Shawm of irregular form.
...
A Clarionet of deeper pitch ... is known in England as the
Corno di Bassetto, and in Germany as the Krumm-horn (crooked-horn).”
The krummhorn, cornet, shawm, and corno di bassetto (basset horn) are all,
of course, distictively different instruments, as are the stops that bear
their names.

Variants

Examples

Osiris contains about 300 examples of Cromorne, about 20 dozen
examples of Krummhorn, 17 of Kromhoorn, 16 of Krumhorn,
14 of Cromhorne, three of Crumhorn, and one of Cormorne.
Most are at 8' pitch,
with a handful at 16' and 4'. The earliest examples are listed below.
See Cremona for examples of that name.
No examples of the names Brumhorn, Brummhorn,
Cornehorne (listed only by Williams),
Cremorne (listed only by Locher), Crommehorne (listed only by Sumner),
or Phocinx are known.
Contributions welcome.

Cromorne 8'; Saint-Gervais, Paris, France; Langhedul? 1601.
According to Douglass, this was the first appearance of this stop in Paris.